Generally, a fuel cell is formed in a structure where a proton conductivity polymer membrane is disposed between a fuel cell electrode (anode) and an oxide electrode (cathode). As shown in FIG. 1, a proton conductivity polymer membrane 11 is formed of a solid polymer with a thickness of 20˜200 μm. The anode and a cathode are formed of a gas spreading electrode (hereinafter, anode and cathode may be referred to as gas spreading electrode) formed of supports 14 and 15 capable of supplying a reaction gas, respectively, and catalyst layers 12 and 13 capable of achieving an oxidation and a reduction of a reaction gas. In FIG. 1, reference numeral 16 refers to a cathode plate having a reaction gas injection groove and having a function of a current collector.
Electricity can be generated in a fuel cell as follows. Hydrogen gas, a fuel gas as shown in the following Reaction 1, is supplied to an anode, which is a fuel electrode and is absorbed by a platinum catalyst, so that proton and electrons are generated based on an oxidation reaction. At this time, the electrons are transferred to the cathode, an oxidation electrode, along the external circuit. The protons are transferred to the cathode through a polymer electrolyte membrane.
In another principle, as shown in the following Reaction 2, the oxygen molecules receive electrons from the cathode and are reduced to oxygen ions. The reduced oxygen ions and protons are reacted for thereby generating electricity.2H2→4H++4e−  [Reaction 1]O2+4e−→2O2−2O2−+4H+→2H2O  [Reaction 2]
Although the fuel cell electrolyte membrane is as an insulator capable of electrically separating the anode and the cathode, it also serves as a medium capable of transferring protons from an anode to a cathode during a cell operation. In addition, it serves to separate a reaction gas or a liquid. Therefore, the fuel cell electrolyte membrane should maintain proton conductivity in a wide range of temperatures, exhibit electrical and chemical stability and should minimize ohmic loss at high current densities. Further, the electrolyte membrane should have good resolution performance of a reaction during a fuel cell operation. A certain level of mechanical properties and dimensional stability also can be required for stacking construction.
The perfluorosulfonic acid resin of Nafion™ that has been widely used as a fuel cell electrolyte membrane has a poly(tetrafluoroethylene) (PTFE) backbone. Nafion™ is a proton exchange resin having sulfonic groups. When over 20 wt % of such a polymer is hydrated, —SO3H of a resin branch is dissociated to thereby obtain higher proton conductivity (˜0.1 S/cm at 25° C.). High mechanical properties, anti-chemical properties and electrical and chemical stability thereby can be obtained.
However, in a thin membrane state in which an area resistance is minimized, dimensional stability, mechanical properties and resolution ability of reaction material are concurrently decreased, which can decrease fuel cell performance.
In addition, since it is possible to restrict platinum catalyst absorption of carbon monoxide included in fuel, the efficiency of a fuel cell system can be enhanced by increasing a reaction speed at high temperatures. Therefore, a high temperature operation method is preferred for development of large or medium size type polymer electrolyte fuel cell for home and electric vehicle use. However, at above 100° C., a rapid increase of membrane resistance of Nafion™ by moisture evaporation allows an operation of proton exchange membrane fuel (PEMFC) to be performed within a boiling point of water. Therefore, development and actual use of a next generation type electrolyte membrane having excellent high temperature conductivity and moisture condition are needed. In order to achieve the above development and actual use, various electrolyte matrixes and organic and inorganic additives have been reported.
U.S. Pat. Nos. 5,547,551, 5,599,614, and 5,635,041 report methods for fabricating reinforced composite membrane (product name: Gore-Select) as an electrolyte membrane where a proton exchange resin in a liquid state is impregnated into extended porous polytetrafluoroethylene (see U.S. Pat. Nos. 3,953,566 and 3,962,153). The fuel cell electrolyte membrane fabricated by the above methods may have a lower conductivity (Ω−1 cm−1) as compared with Nafion™, but these materials maintain a desired mechanical strength and dimensional stability of a porous polymer support for thereby achieving a thin membrane with a size of about 25 μm. The conductivity (Ω−1 cm−2) of such a fabricated composite electrolyte membrane is considered to be excellent as compared with Nafion™.
However, in such a fabricated composite polymer electrolyte membrane, the thickness of the membrane is decreased to about 25 μm in order to enhance the ion conductivity, so that tear strength becomes relatively low. In this case, it is necessary to fully impregnate expensive porous polytetrafluoroethylene support with an 80% porous ratio into a Nafion™ resin. This results in increased fabrication costs. In addition, since the proton exchange resin must be repeatedly impregnated into polytetrafluoroethylene film with a relatively low wettability, the fabrication process is performed rather slowly and non-continuously.
U.S. Pat. No. 5,525,436 reports method for fabricating an electrolyte membrane where a solvent is evaporated from polybenzimidazole solution and is doped with a strong acid such as sulfuric acid and dried. In U.S. Pat. Nos. 5,091,087, 5,599,639 and 6,187,231, polyamide is coated on polybenzimidazole, and a composite film is fabricated by a compression molding method. Thereafter, polyamide is extracted using a solvent such as dichloromethane, and then a porous polybenzimidazole film is fabricated. An electrolyte membrane may be fabricated by performing a doping process using strong acid. In another method, polybenzimidazole solution doped with strong acid is solidified in a liquid bath with a non-solvent or a mixture of a non-solvent and a solvent for thereby fabricating an electrolyte membrane.
In addition to the above methods, there are electrolyte membranes fabricated where a polymer such as polyphosphazene (WO 00/77874), polyethersulfon (Japan patent laid-open No. 11-116679), polyether-etherketone and poly(4-phenoxybenzoyl-1,4-phenylene) is sulfonated and added with organic or inorganic ion conductive agents.
In a certain method for maintaining a desired conductivity of an electrolyte membrane at high temperatures, the use of an organic or inorganic hydrophilic additive having an excellent coupling force has been attempted. It has been reported that an ion or dipole of a heteropoly acid compound of phosphotungstic acid (PTA) is strongly coupled with proton and inhibits moisture evaporation at high temperature. [S. Malhotra, R. Datta, J. Electrochem. Soc., 144 L23 (1977)]. However, that PTA is a water soluble substance, so that it may be extracted to the outside of a fuel cell due to mass transport of moisture generated during a cell operation.
It also has been reported to substitute moisture with a proton acceptor in an electrolyte membrane using an organic solvent having a low volatility. The moisture has a high dielectric constant as Bronsted base, so that it can easily dissociate —SO3H. Since moisture is a by-product of a fuel cell reaction, it is necessary to use moisture with a proton exchange membrane. At present, certain studies for fabricating an electrolyte membrane comprising phosphoric acid, imidazole, butyl methyl imidazolium triflate, butly methyl imidazolium tetra fluoro bonate, etc. are under progress. [1: R. Savinell, et al., J. Electrochem. Soc., 141, L46 (1994); 2: K. D. kreuer, A. Fuchs, M. Ise, M. Sapeth. J. Mater. Electrochem. Acta, 43, 1281 (1998)].
A water non-soluble solid proton conductor of ion transfer without moisture has been investigated. Workers in the United States and Japan as well as in other countries have concentrated their studies on fabricating a composite electrolyte membrane (CsHSO4, Zr(HPO4)2) that is added to a proton exchange resin. [S. M. Haile, D. A. Yoysen, C. R. I. Chisolm, R. B. Merle, Nature, 410. 910(2001)]
In attempted methods where CsHSO4 or Zr(HPO4)2 is admixed in a Nafion™ solution and the solvent is dried, a large amount of inorganic additive of a high density is non-uniformly distributed on the Nafion™ membrane. An adhering force with respect to a polymer matrix becomes low, and brittleness is increased. In a composite electrolyte membrane method according to U.S. Pat. No. 5,919,583, a Nafion™ membrane is swelled in ZrOCl3 water solution and is processed with phosphoric acid solution, so that a fine powder type Zr(HPO4)2 is formed in Nafion™ membrane.
However, in that case, the dissociated Zr4+ ion is formed in a hydrophilic region of hydrated Nafion™ membrane or a surface region of the same, and the added amount of Zr(HPO4)2 is limited to about 20 wt %, thus limiting the formation of ion conduction mechanism based on a physical contact with inorganic additives.
The information set forth in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.